Lecture 10 ANNOUNCEMENTS. The post lab assignment for Experiment #4 has been shortened! 2 pgs of notes (double sided, ) allowed for Midterm #1

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1 Lecture 0 ANNOUNCMNTS Alan Wu will hold an extra lab session tomorrow (9/28), 2 4PM The post lab assignment for xperiment #4 has been shortened! 2 pgs of notes (double sided, 8.5 ) allowed for Midterm # OUTLIN BJT Amplifiers (cont d) CB stage with biasgi mitter follower (Common collector amplifier) Analysis of emitter follower core Impact of source resistance Impact of arly effect mitter follower with biasg eadg: Chapter Fall 2007 Lecture 0, Slide Prof. Liu, UC Berkeley

2 Biasg of CB Stage is necessary to proide a path for the bias current I to flow, but it lowers the put impedance. g g gm m gm X S A out out X X out gm C g m g ( ) m ( m ) S S 05 Fall 2007 Lecture 0, Slide 2 Prof. Liu, UC Berkeley

3 eduction of Input Impedance Due to The reduction of put impedance due to i is undesirable because it shunts part of the put current to ground stead of to Q (and C ). Choose >> /g m, i.e. I C >> V T 05 Fall 2007 Lecture 0, Slide 3 Prof. Liu, UC Berkeley

4 Creation of V b A resistie oltage diider lowers the ga. To remedy this problem, a capacitor is serted between the base and ground to short out the resistie oltage diider at the frequency of terest. 05 Fall 2007 Lecture 0, Slide 4 Prof. Liu, UC Berkeley

5 xample of CB Stage with Bias Design a CB stage for A 0 and 50Ω. 50Ω /g m if >> /g m Choose 500Ω V CC 2.5V I S 5x0-6 A β 00 V A A g m C 0 C 500Ω I C g m V T 0.52mA V B V T ln(i C /I S )0.899V V b I V B.6V Choose and 2 to proide V b and I >> I B, e.g. I 52µA C B ischosen so that (/(β))(/ωc B ) is smallcompared to /g m at the frequency of terest. 05 Fall 2007 Lecture 0, Slide 5 Prof. Liu, UC Berkeley

6 mitter Follower (Common Collector Amplifier) 05 Fall 2007 Lecture 0, Slide 6 Prof. Liu, UC Berkeley

7 mitter Follower Core When the put oltage (V ) is creased by V, the collector current (and hence the emitter current) creases, so that the output oltage (V out ) is creased. Note that V and V out differ by V B. 05 Fall 2007 Lecture 0, Slide 7 Prof. Liu, UC Berkeley

8 Unity Ga mitter Follower In tegrated circuits, the follower is typically realized asshown shown below. The oltage ga is because a constant collector current ( I ) results a constant V B ; hence V out V. V A A 05 Fall 2007 Lecture 0, Slide 8 Prof. Liu, UC Berkeley

9 Small Signal Model of mitter Follower The oltage ga is less than and positie. V A π out π KCLat emitter: g r r π out g m m π π out ( ) out out rπ β g m out 05 Fall 2007 Lecture 0, Slide 9 Prof. Liu, UC Berkeley

10 mitter Follower as a Voltage Diider V A 05 Fall 2007 Lecture 0, Slide 0 Prof. Liu, UC Berkeley

11 mitter Follower with Source esistance V A out g m S β 05 Fall 2007 Lecture 0, Slide Prof. Liu, UC Berkeley

12 Input Impedance of mitter Follower The put impedance of an emitter follower is the same as that of a C stage with emitter degeneration (whose put impedance does not depend on the resistance between the collector and V CC) ). V A x r π ( β ) ix 05 Fall 2007 Lecture 0, Slide 2 Prof. Liu, UC Berkeley

13 ffect of BJT Current Ga There is a current ga of (β) from base to emitter. ffectiely, the load resistance seen from the base is multiplied by (β). 05 Fall 2007 Lecture 0, Slide 3 Prof. Liu, UC Berkeley

14 mitter Follower as a Buffer The emitter follower is suited for use as a buffer between a C stage and a small load resistance, to alleiate the problem of ga degradation. A g m ( ) C speaker rπ 2 β2) A g m ( ( ) C speaker 05 Fall 2007 Lecture 0, Slide 4 Prof. Liu, UC Berkeley

15 Output Impedance of mitter Follower An emitter follower effectiely lowers the source impedance by a factor of β, β, for improed drig capability. The follower is a good oltage buffer because it has high hput impedance and low output impedance. s out gm β 05 Fall 2007 Lecture 0, Slide 5 Prof. Liu, UC Berkeley

16 mitter Follower with arly ffect Sce r O is parallel with, its effect can be easily corporated to the equations for the oltage ga and the put and output impedances. A r π ro S r O β g ( β )( r ) s out r β gm 05 Fall 2007 Lecture 0, Slide 6 Prof. Liu, UC Berkeley O m O

17 mitter Follower with Biasg A biasg technique similar to that used for the C stage can be used for the emitter follower. Note that V B can be biased to be close to V CC because the collector is biased at V CC. 05 Fall 2007 Lecture 0, Slide 7 Prof. Liu, UC Berkeley

18 Supply Independent Biasg By puttg an dependent current source at the emitter, the bias pot (I C, V B ) is fixed, regardless of the supply oltage alue. 05 Fall 2007 Lecture 0, Slide 8 Prof. Liu, UC Berkeley

19 Summary of Amplifier Topologies The three amplifier topologies studied thus far hae different properties p and are used on different occasions. C and CB stages hae oltage ga with magnitude greater than one; the emitter follower s oltage ga is at most one. 05 Fall 2007 Lecture 0, Slide 9 Prof. Liu, UC Berkeley

20 Amplifier xample # The keys to solg this problem are recognizg the AC ground between and 2, and usg a Theen transformation of the put network. C stage Small-signal Simplified small-signal equialent circuit equialent circuit out S β 2 C g m S 05 Fall 2007 Lecture 0, Slide 20 Prof. Liu, UC Berkeley

21 Amplifier xample #2 AC groundg/shortg and Theen transformation areneeded to transform this complex circuit to a simple C stage with emitter degeneration. out C S 2 β g 05 Fall 2007 Lecture 0, Slide 2 Prof. Liu, UC Berkeley m S

22 Amplifier xample #3 First, identify eq, which is the impedance seen at the emitter of Q 2 parallel with the fite output impedance of an ideal current source. Second, use the equations for a degenerated C stage with ih replaced dby eq. C A g g 2 β m m eq g m 2 β 05 Fall 2007 Lecture 0, Slide 22 Prof. Liu, UC Berkeley r π r π 2

23 Amplifier xample #4 Note that C B shorts out 2 and proides a ground for, at the frequency of terest. appears parallel with C ; the circuit simplifies to a simple CB stage with source resistance. A C g m S 05 Fall 2007 Lecture 0, Slide 23 Prof. Liu, UC Berkeley

24 Amplifier xample #5 Note that the equialent base resistance of Q is the parallel connection of and the impedance seen at the emitter of Q 2. S gm gm β 2 β 05 Fall 2007 Lecture 0, Slide 24 Prof. Liu, UC Berkeley